How to manually build Expression which will return always true?

Question

I tried to create Expression, but failed.

I want to build something like Expression<Func<typeof(type), bool>> expression = _ => true;

My attempt:

private static Expression GetTrueExpression(Type type)
{
    LabelTarget returnTarget = Expression.Label(typeof(bool));
    ParameterExpression parameter = Expression.Parameter(type, "x");

    var resultExpression = 
      Expression.Return(returnTarget, Expression.Constant(true), typeof(bool));

    var delegateType = typeof(Func<,>).MakeGenericType(type, typeof(bool));

    return Expression.Lambda(delegateType, resultExpression, parameter); ;
}

Usage:

var predicate = Expression.Lambda(GetTrueExpression(typeof(bool))).Compile();

But I am getting the error: Cannot jump to undefined label ''

You might try using `PredicateBuilder` by the Albahari Brothers. `var expr = PredicateBuilder.True();` http://www.albahari.com/nutshell/predicatebuilder.aspx — willaien, Apr 21 '16 at 15:02
`typeof(type)` in your expression will *always* return `Type` — maraaaaaaaa, Apr 21 '16 at 15:08
This works too: `public static Expression GetTrueExpression() => Expression>)(x => true); var predicate = GetTrueExpression — 15ee8f99-57ff-4f92-890c-b56153, Apr 21 '16 at 15:27

score 18 · Accepted Answer · answered Apr 21 '16 at 15:02

18

As simple as that:

private static Expression GetTrueExpression(Type type)
{
    return Expression.Lambda(Expression.Constant(true), Expression.Parameter(type, "_"));
}

answered Apr 21 '16 at 15:02

Ivan Stoev

195,425
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score -1 · Answer 2 · answered Mar 23 '20 at 15:21

-1

I had to face the same issue, and got this code. It seems to be clean and simple.

Expression.IsTrue(Expression.Constant(true))

answered Mar 23 '20 at 15:21

Ygalbel

5,214
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I have no idea. I think it's because my answer don't take a parameter. – Ygalbel Apr 30 '20 at 20:11
2

I guess because OP is looking for a "callable generic function Expression" and this is just a "True" expression. Ivan Stoev's answer is clever. Creates an expression that represents a lambda, when you call the lambda expression with any type of parameter (just one) always returns true. – dani herrera Apr 30 '20 at 20:17

How to manually build Expression which will return always true?

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